Source : Webster's Revised Unabridged Dictionary (1913)

Atresia \A*tre"si*a\, n. [NL., fr. Gr. ? not perforated.] (Med.)
   Absence or closure of a natural passage or channel of the
   body; imperforation.

Source : WordNet®

atresia
     n : an abnormal condition in which a normal opening or tube in
         the body (as the urethra) is closed or absent